1. Field of the Invention
The present invention relates to a bonding structure between electronic device packages. More particularly, the present invention relates to a bump structure that can provide good bonding properties.
2. Description of the Related Art
In the fabrication of high-density electronic packages, a means of enhancing the bonding effect between an integrated circuit device and a carrier substrate, thereby increasing the production yield, is always an important research topic.
Using liquid crystal display (LCD) as an example, the technique for packaging an LCD has changed from chip-on-board (COB) to tape-automated-bonding (TAB) and then to the current fine pitch chip-on-glass (COG) due to the need for higher image resolution and the demand for a lighter and slimmer electronic product.
However, in most conventional packaging process that uses bumps as a means of bonding, the difference in the coefficient of thermal expansion (CTE) between the chip and the carrier substrate is quite significant. Therefore, after the chip and the carrier substrate are bonded together, warpage often occurs due to CTE mismatch between the chip, the bumps and the carrier substrate. As a result, the bumps are thermally stressed. Moreover, with the ever-increasing level of integration of the integrated circuit, the effects resulting from the thermal stress and the warpage are increasingly significant. One of the major effects includes a drop in the reliability of connection between the chip and the carrier substrate and the subsequent failure to comply with the reliability test.
K. Hatada in U.S. Pat. No. 4,749,120 proposed using gold bumps to serve as an electrical connection between a chip and a substrate, and in the meantime, using resin as a bonding agent between the two. However, the Young's modulus of metal is substantially higher than resin. Hence, in the process of joining the chip and the carrier substrate together and curing the resin, considerable contact stress must be applied. In addition, the gold bumps will be subjected to considerable peeling stress after the bonding process so that the gold bumps may peel off from the chip or the carrier substrate.
In another method, Y. Tagusa et. al in U.S. Pat. No. 4,963,002 proposed using nickel-plated (nickel) beads or silver particles to achieve electrical connection. Yet, this method is only suitable for bonding a small area. Furthermore, if the silver particles are used in the bonding process, the large Young's modulus of silver may lead to the same bump-peeling problem.
In yet another method, Sokolovsky et. al in U.S. Pat. No. 4,916,523 proposed using a unidirectional conductive bonding agent to bond the chip and the carrier substrate together. On the other hand, Brady et. al in U.S. Pat. No. 5,134,460 also proposed a design that involves coating a metallic layer over conductive metal bumps.